Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermographic materials of type 1 become photothermographic when a photosensitive agent is present which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density. Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
In U.S. Pat. No. 5,441,866 it is stated that: "While many of such dyes (dyes which impart spectral sensitivity to a gelatino silver halide element) provide spectral sensitization in photothermographic formulations the dye sensitization is often very inefficient and it is not possible to translate the performance of a dye in gelatino silver halide elements to photothermographic elements."
In the context of infra-red sensitization of "Dry Silver" materials, EP-A 559 228 discloses a photothermographic emulsion comprising a binder, a light insensitive silver salt, a reducing agent for silver ion, and silver halide, the silver halide being spectrally sensitized to radiation of from 750 to 1300 nm and the emulsion containing a supersensitizing amount of a compound selected from the group consisting of heteroaromatic mercapto compounds or heteroaromatic disulfide compounds.
Typical supersensitizers are either dyes or colorless compounds showing strong absorption bands in the near-ultraviolet spectral region associated with electron systems similar to those of the dyes. Sensitization enhancement of a particular spectral sensitizer or class of spectral sensitizers is often only observed with particular compounds or for a narrow class of compounds. Therefore, it is important that the range of compounds available for enhancing the IR-sensitivity of photothermographic recording materials be as large as possible.